Siesta or Nap
The siesta habit has recently been associated with a 37% reduction in coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep. Nevertheless, epidemiological studies on the relations between cardiovascular health and siesta have led to conflicting conclusions, possibly because of poor control of moderator variables, such as physical activity. It is possible that people who take a siesta have different physical activity habits, e.g., waking earlier and scheduling more activity during the morning. Such differences in physical activity may mediate different 24-hour profiles in cardiovascular function. Even if such effects of physical activity can be discounted for explaining the relationship between siesta and cardiovascular health, it is still unknown whether it is the daytime nap itself, a supine posture or the expectancy of a nap that is the most important factor. It was recently suggested that a short nap can reduce stress and blood pressure (BP), with the main changes in BP occurring between the time of lights off and the onset of stage 1. Dr. Zaregarizi and his team have concluded that the acute time of falling asleep was where beneficial cardiovascular changes take place. This study has indicated that a large decline in blood pressure occurs during the daytime sleep-onset period only when sleep is expected however when subjects rest in a supine position, the same reduction in blood pressure is not observed. This blood pressure reduction may be associated with the lower coronary mortality rates seen in Mediterranean and Latin American populations where siestas are common. Dr. Zaregarizi assessed cardiovascular function (blood pressure, heart rate, and measurements of blood vessel dilation) while nine healthy volunteers, 34 years of age on average, spent an hour standing quietly; reclining at rest but not sleeping; or reclining to nap. All participants were restricted to 4 hours of sleep on the night prior to each of the sleep laboratory tests. During the three phases of daytime sleep, he noted significant reductions in blood pressure and heart rate. By contrast, they did not observe changes in cardiovascular function while the participants were standing or reclining at rest. These findings also show that the greatest decline in blood pressure occurs between lights-off and onset of daytime sleep itself. During this sleep period, which lasted 9.7 minutes on average, blood pressure decreased, while blood vessel dilation increased by more than 9 percent. “There is little change in blood pressure once a subject is actually asleep," Dr. Zaregarizi noted, and he found minor changes in blood vessel dilation during sleep.
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